Update information regarding new runner creation workflow

What does this MR do?

This MR updates the removal milestone and adds links to https://docs.gitlab.com/ee/ci/runners/new_creation_workflow in a couple of warning messages:

• When a runner authentication token is specified in --registration-token.
• When deprecated registration arguments are specified together with a runner authentication token.

Why was this MR needed?

To reduce some unnecessary confusion for users. Example: https://gitlab.slack.com/archives/C16HYA2P5/p1698159539116419 (internal)

What's the best way to test this MR?

N/A

What are the relevant issue numbers?

• gitlab#429535 (closed)
• gitlab#429269 (closed)